Change libpng version to 1.2.15 especially to fix security issue CVE-2006-3334.

Tested on epsilon, veda, altair/mingw32.

extras/contrib/src/packages.mak

@@ -118,7 +118,7 @@ VCDIMAGER_VERSION=0.7.23
 VCDIMAGER_URL=$(GNU)/vcdimager/vcdimager-$(VCDIMAGER_VERSION).tar.gz
 CDIO_VERSION=0.78.2
 CDIO_URL=$(GNU)/libcdio/libcdio-$(CDIO_VERSION).tar.gz
-PNG_VERSION=1.2.12
+PNG_VERSION=1.2.15
 PNG_URL=$(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 GPGERROR_VERSION=1.3
 GPGERROR_URL=http://ftp.gnupg.org/gcrypt/libgpg-error/libgpg-error-$(GPGERROR_VERSION).tar.gz